Add missing test case for contravariant trait matching

Author: nikomatsakis

Diff for: src/test/run-pass/trait-contravariant-self.rs

@@ -0,0 +1,37 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This is an interesting test case. We have a trait (Bar) that is
+// implemented for a `Box<Foo>` object (note: no bounds). And then we
+// have a `Box<Foo:Send>` object. The impl for `Box<Foo>` is applicable
+// to `Box<Foo:Send>` because:
+//
+// 1. The trait Bar is contravariant w/r/t Self because `Self` appears
+//    only in argument position.
+// 2. The impl provides `Bar for Box<Foo>`
+// 3. The fn `wants_bar()` requires `Bar for Box<Foo:Send>`.
+// 4. `Bar for Box<Foo> <: Bar for Box<Foo:Send>` because
+//    `Box<Foo:Send> <: Box<Foo>`.
+
+trait Foo { }
+struct SFoo;
+impl Foo for SFoo { }
+
+trait Bar { fn dummy(&self); }
+impl Bar for Box<Foo> { fn dummy(&self) { } }
+
+fn wants_bar<B:Bar>(b: &B) { }
+
+fn main() {
+    let x: Box<Foo:Send> = (box SFoo);
+    wants_bar(&x);
+}
+
+